test: improve test conventions

Specifically, functions that are run in the context of the test runner
are put in module `test/testutil.lua` while the functions that are run
in the context of the test session are put in
`test/functional/testnvim.lua`.

Closes https://github.com/neovim/neovim/issues/27004.

test/functional/provider/perl_spec.lua

@@ -1,14 +1,16 @@
-local t = require('test.functional.testutil')()
-local eq, clear = t.eq, t.clear
-local missing_provider = t.missing_provider
-local command = t.command
+local t = require('test.testutil')
+local n = require('test.functional.testnvim')()
+
+local eq, clear = t.eq, n.clear
+local missing_provider = n.missing_provider
+local command = n.command
 local write_file = t.write_file
-local eval = t.eval
+local eval = n.eval
 local retry = t.retry
-local api = t.api
-local insert = t.insert
-local expect = t.expect
-local feed = t.feed
+local api = n.api
+local insert = n.insert
+local expect = n.expect
+local feed = n.feed
 
 do
   clear()